fix(studio): responsive sidebar — collapse to top nav on mobile

[christso]

Problem

The Studio sidebar is fixed at w-64 (256px) with no responsive breakpoints. On viewports narrower than ~800px the sidebar pushes the main content area off-screen and the content is unreadable.

Observed during visual verification of the comparison matrix (#990): on a 390×844 viewport the sidebar occupies the left half of the screen, leaving ~134px for main content.

Affected file: apps/studio/src/components/Sidebar.tsx (unchanged since #977)

Proposed fix

On md and above: keep the current fixed sidebar.
Below md: hide the sidebar and show a hamburger toggle that slides it in as an overlay, or collapse navigation into a top bar.

The simplest option:

• Add hidden md:flex to the <aside> (hides on mobile)
• Add a mobile top bar with a hamburger that toggles sidebar visibility via state

Acceptance criteria

• On viewports ≥ 768px: sidebar renders as today (no visual change)
• On viewports < 768px: main content fills the screen; navigation accessible via a toggle
• Comparison matrix, run detail, and project list are readable on a 390px viewport